<!DOCTYPE html>
<html lang="ch">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }

        .box {
            width: 500px;
            height: 500px;
            background-color: yellow;
            /* margin: 0 auto;  水平方向居中 */
            /* margin: 0 auto; */
            /* position: absolute;
            top: 50%;
            left: 50%;
            margin-top: -100px;
            margin-left: -100px; */
            margin: 0 auto;
            position: relative;
        }

    .child{
        width: 200px;
        height: 200px;
        background-color: red;
        /* margin: 0 auto; 水平居中 */
        position: absolute;
        left: 50%;
        top: 50%;
        margin-top: -100px;
        padding-left: -100px;
    }
    </style>
</head>

<body>
    <div class="box">

        <div class="child">

        </div>

    </div>
</body>

</html>